While the counterbalance system of the present invention is applicable to many types of vertical movable means, it is particularly well adapted as a counterbalance mechanism for the vertically movable spindle carrier of a machine tool such as a numerically-controlled machining center, or the like. As a consequence for purposes of any exemplary embodiment, the counterbalance system will be described in terms of its application to the spindle carrier of a machine tool.
In one of its most common forms, the spindle carrier of a machining center, or the like, is shiftable vertically upwardly and downwardly along ways by a ballscrew assembly operatively connected to the spindle carrier and actuated by a drive motor. A ball screw is typically mounted to the column of the machine tool and engages a ball nut attached to the spindle carrier. Since a spindle carrier can be extremely heavy, weighing 24,000 pounds or more, it is desirable to provide a counterbalance system for the spindle carrier to assist the drive motor in moving the spindle carrier. This also minimizes the compression of the ballscrew assembly and increases its life by reducing the mechanical stress within the material of the ball screw assembly. A numerically-controlled machine tool has a spindle carrier movable along a vertical or Y axis and which is typically powered by an electric drive motor or some other type of motor means. The spindle carrier is movable along vertical ways of the machine tool and is connected to the electric drive motor through a relatively long axially-fixed ballscrew having a length such as one hundred inches, for example. The spindle carrier may weigh 24,000 pounds, for example, thus placing a relatively large load on the ballscrew. This load can stretch and/or compress the ballscrew, which is mounted to a ball nut that may travel the length of the ballscrew, and thus interfere with the operation and degrade the accuracy of the machine tool.
To minimize the stretch or compression of the ballscrew, and to assist the drive motor in moving the spindle carrier, a counterbalance mechanism is used with the spindle carrier. U.S. Pat. No. 4,807,518 describes one such prior counterbalance mechanism which has the spindle carrier attached to one end of a chain which is threaded around sprockets and secured at its other end to a fixed structure. The other end of the chain is attached to a piston rod of a piston slidable within a hydraulic cylinder. By applying hydraulic fluid under pressure to the end of the piston attached to the piston rod, a counterbalance force is applied to the spindle carrier. This counterbalance mechanism enables the spindle carrier to be more easily moved by the electric drive motor. However, the inventor's of U.S. Pat. No. 4,807,518 found that when the spindle carrier was moved downwardly by the electric drive motor, the upward movement of the piston within the cylinder caused an increase in the hydraulic pressure causing compression of the ballscrew and when the spindle carrier was moved upwardly by the electric drive motor, the downward movement of the piston in the hydraulic cylinder caused a decrease in the hydraulic pressure thereby producing tension in the ballscrew. With a typical ballscrew having a length of about one hundred inches, extension or contraction of the ballscrew due to such tension or compression respectively can substantially affect the position to which the spindle carrier is moved by the electric drive motor. The relative force locations within a horizontal plane through the carrier (i.e. the ball screw force), the counterbalance force, the slideway bearing friction force, and the force of the carrier's center of gravity, could result in a moment which will change during the transition from upward to downward carrier motion. This changing moment causes the carrier to rotate or rock depending upon the magnitude of the moment and the compliance of the slideway bearing supporting the carrier. This phenomena will substantially influence the positioning accuracy or lost motion at the machine's tool tip. If it wasn't for this changing moment, a servo slide feedback system could solve the resulting positioning problem. Thus, accurate and precise positioning of the spindle carrier, typically within 0.0005" for example, cannot easily be obtained, if at all. This counterbalance system also has deadband problems at spindle carrier reversal.
Thus the inventor's of U.S. Pat. No. 4,807,518, citing these reasons as well as others such as hydraulic and mechanical inefficiencies including the friction of the chain at the sprockets, movement of the spindle carrier along the ways, and the hydraulic piston sliding in the cylinder, proposed an apparatus which produces a substantially constant counterbalance force. Such an apparatus uses a load cell to produce a signal indicative of the sensed load on the spindle carrier and a control means which uses the signal to vary the hydraulic pressure applied to a piston within the hydraulic cylinder which provides the counterbalance force. The system of U.S. Pat. No. 4,807,518 has been used and has been found to substantially reduce and eliminate deadband problems at spindle carrier reversal and can compensate for the counterbalance valve and line pressure losses at high slide speeds or hydraulic flow rates, conditions which cause problems with traditional counterbalance systems not having load cell feedback.
A further improvement to this system included an accumulator for the hydraulic piston and cylinder provided a faster response and requires less hydraulic power to operate the system. This resulted in considerable savings in operating energy and expenses, reduced noise, and a requirement for a smaller pump and tank. An example of a machine tool is the High Performance CNC Machining center with ACRAMATIC 850MC Computer Numerical Control which has an accumulator as illustrated in its parts and service manual (1987) on page 2-1-3. Accumulator counterbalance systems, particularly these utilizing gas-charged accumulators are, however, characterized by varying counterbalance pressure or force, as the slide moves over its length of travel, particularly for large machines. As a consequence, the accumulator counterbalance system is most useful where the spindle carrier has a short vertical travel and is relatively lightweight. Under these circumstances, pressure or counterbalance force variations are often negligible or within acceptable limits.
The present invention is based upon the discovery that excellent results can be achieved through the use of a two-part system utilizing portions of the known cylinder and accumulator hydraulic system to provide the major counterbalancing force to a chain, together with a high speed torque controlled servomotor operatively attached to one of the chain sprockets to correct or compensate for any deficient counterbalance force of the accumulator hydraulic system.